Method of producing a cold-rolled strip with a ferritic structure

ABSTRACT

A method of making a cold strip with a ferritic structure includes casting a molten steel which forms a ferritic structure on cooling into a cast strip, wherein if necessary hot-rolling the cast strip in-line, coiling the hot-rolled strip and in one or more steps cold-rolling to form the cold strip. With such a method, cold strips can be produced, in which the risk of formation of an orange peel appearance or ridging is minimized during a cold forming process. The cast strip is cooled between the casting process and the coiling process from a starting temperature not lower than 1180° C., at a cooling rate of at least 150° C. per second, to a maximum intermediate temperature of 1000° C. and is then held for at least 10 seconds at a maintenance temperature of between 900 and 1000° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Application of International Application No. PCT/EP2006/070223, filed on Dec. 27, 2006, which claims the benefit of and priority to German patent application no. DE 10 2005 063 058.8, filed Dec. 29, 2005. The disclosure of each of the above applications is incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to a method for producing a cold strip with a ferritic structure, in which molten steel which forms a ferritic structure on cooling is cast into a cast strip, wherein if necessary the cast strip is hot-rolled in-line, the hot-rolled strip is coiled or wound and in one or more steps is cold-rolled to form the cold strip.

BACKGROUND

Due to the high price of nickel, austenitic stainless steel is increasingly being substituted worldwide by ferritic stainless steel, which contains Ni typically only as a production-related companion element. A method of the type initially indicated, which is to make this possible, is known for example from the European Patent EP 0 881 305 B1. According to the prior art method a stainless, ferritic steel, which contains (in wt. %) max. 0.12% C, max. 1% Mn, max. 1% Si, max. 0.04% P, max. 0.030% S, 16-18% Cr and as remainder iron and unavoidable impurities, is cast into strip in the casting gap formed between the rolls of a twin roll casting machine. Subsequently, the cast strip is cooled down, during which cooling process holding the strip in the austenite-ferrite transformation range is avoided. After cooling, the strip is coiled or wound at a temperature, which lies between 600° C. and the martensite transformation temperature.

Subsequently, the coiled or wound strip is cooled to a temperature, which lies between 200° C. and ambient temperature, at a maximum rate of 300° C. per hour. Finally, batch type annealing, known per se, of the wound strip is carried out.

With the route via the continuous casting of slabs, normally taken for the production of ferritic steel sheet, firstly the surface of the slab is shaped, then the slab is re-heated, afterwards the slab is hot-rolled in the hot strip mill into hot strip and then wound to a coil. The hot strip obtained in this way is thereupon annealed, pickled and cold-rolled in several passes. Finally, the cold strip is normally bright-annealed and skin-pass rolled.

Regardless which of the ways mentioned above is used to produce the hot strip, the problem with cold-rolled strip made from ferritic stainless steel with a Cr content in the range of 17% is that ridging or orange peel can form in the course of subsequent cold working, particularly deep-drawing. Ridging here describes strongly marked linear surface defects, which in the case of ferritic chromium steel are aligned in the rolling direction. On the other hand, the surface defect described as “orange peel” is non-directional and is marked by rough appearance of the surface.

The formation of ridging or orange peel can be avoided if at extra expense the hot strip produced via one of the known production routes is intermediately annealed between the individual cold-rolling passes. These expensive annealing steps, however, lead to increased production costs which result in a higher market price of ferritic stainless steel strip compared to equivalent material made from austenitic stainless steel.

SUMMARY OF THE INVENTION

In one embodiment, the invention features a method with which a cold-rolled strip can be produced from ferritic stainless steel, in which the risk of formation of orange peel or ridging is minimized during a cold-forming process.

In one embodiment of the method of making the cold-rolled strip, a strip cast from molten metal is cooled between a casting process and a coiling process from a starting temperature not lower than 1180° C., at a cooling rate of at least 150° C. per second, to a maximum intermediate temperature of 1000° C. and is then held for at least 10 seconds at a maintenance temperature of between 900 and 1000° C.

Typical starting temperatures of high-intensity cooling typically range from 1180 to 1270° C., particularly 1200 to 1250° C. If the minimum temperature of 1180° C. is not reached, austenite can occur at the strip edge already in quantities, which would impair the success of the method according to the invention.

Particularly suitable for implementing the method according to the invention is such steel, which belongs to the category of stainless steel containing 10-18 wt % Cr forming a ferritic structure and in the course of cooling starting from the ferrite does not completely transform into austenite and then again into ferrite. Such steel, beside iron and unavoidable impurities, typically contains (in wt. %) up to 0.08% C, 10-18% Cr, up to 1% Si, up to 1.5% Mn, up to 1% Ni, up to 0.04% P and up to 0.015% S. The Ni-content possibly present here is not a metallurgically purposeful addition but [[is]] rather can be the result of the production process and enters the molten steel via the Ni-containing foundry ladles, converters or furnaces. Typically the Ni-content of steel processed according to the invention ranges from 0.7 to 0.8 wt. %.

The effectiveness of the method according to the invention can be enhanced by the combination of strip casting, rapid cooling and holding of the cast strip over a sufficient period of 10 seconds at the minimum at a temperature in the range of 950±50° C., particularly 950±20° C. Surprisingly, it has been shown that neither ridging nor orange peel occurs during cold forming of cold strip which has been produced from cast strip of the kind thermally treated according to the invention, without expensive intermediate annealing between the cold-rolling stages having to be carried out for this.

Steel alloys used according to the invention initially solidify ferritically during the course of strip casting. When the solidified strip cools down, ferrite then transforms between 1200° C. and 800° C. partly into austenite. The thermodynamic cause lies in the very low solubility, decreasing with the temperature, of carbon in the ferrite. Carbides, which can also absorb carbon, only form below 900° C. On the other hand the solubility of carbon in the austenite is substantially higher.

During normal cooling, austenite forms at the grain boundaries, since carbon in the ferrite again rapidly diffuses and can migrate to the edges from the grain interior. Thus, the ferrite grain boundaries are marked with austenite. As soon as carbides form at temperatures of less than 900° C., the austenite portion at the grain boundaries again decreases. Due to the relatively slow process of carbide formation, this however does not take place completely, so that austenite residues remain, which in the temperature range of 200-300° C. subsequently transform into martensite. The residual austenite remaining at the grain boundaries with conventional working methods thus produces the rough cast structure.

The idea on which the invention is based now consists of very rapid cooling to approximately the temperature with the maximum austenite portion (950° C.±50° C., particularly 950° C.±20° C.). In this way, austenite formation at the grain boundaries is minimized, since in this case the diffusion distances covered are not sufficient for carbon and just for the substitutional elements (Cr, Ni, Mn, etc.) during the short cooling period.

At the same time the force driving the austenite formation is greatest in the maintenance temperature range of approximately 950° C. and the temperature-dependent diffusion coefficient is so low that austenite particles form in the grain interior by way of nucleation. Due to the substantially reduced diffusion coefficient, the distribution of the substitutional elements contained in the respective alloy does not vary or varies to a small degree (para-equilibrium). At the same time the carbon supersaturation is diminished.

If the cast strip material in the way according to the invention is held at this temperature for a minimum period of ten seconds, preferably twenty seconds, austenite particles thus start to precipitate inside the grain interior at structural defects. New grains, which break up the original cast structure, develop in a ferritic matrix. The particle density becomes greater, the longer the maintenance period. This precipitation mechanism causes grain refinement, which results in the insensitivity of cold strip, produced according to the invention, to ridging and the formation of orange peel.

The higher the cooling rate, during high-intensity cooling of the cast strip according to the invention, the more reliably austenite formation is suppressed. In principle therefore highest possible cooling rates are strived for. However, the effects exploited by the invention can reliably occur at cooling rates of 150-250° C. per second.

DETAILED DESCRIPTION OF THE INVENTION

Different variants of the invention, which can be selected depending upon the desired characteristic combinations of the cold strip to be produced in each case, the hot-forming behavior of the cast strip obtained in each case, the plant technology available or the demands of the operating logistics, result on the basis of the fundamental methods according to the invention explained above. Thus, for example, high-intensity cooling can come first and then hot-rolling or initial rolling (above 1200° C. in the ferrite) and then rapid cooling. Furthermore the two-phase range between 1200 and 800° C. can also be passed through very quickly. Then no austenite forms initially, but ferrite supersaturated with carbon becomes frozen in. If heating is carried out from below 800° C. to the maintenance temperature, again austenite forms in the grain interior. Particularly rapid re-heating to the maintenance temperature in this case positively affects the working result. If heating is carried out too slowly, unwanted austenite can form at the grain boundaries, while the carbon supersaturation in the grain interior is diminished by carbon diffusion, which results in the original cast structure being fixed. Moreover, holding in the temperature range of 800-900° C. for too long should be avoided, because in this temperature range, after approximately 100 seconds, the carbon supersaturation is diminished by carbide formation.

Carbide can no longer form if, during the course of cooling, temperatures of less than 500° C., ambient temperature for example, are reached. On the contrary, the supersaturated ferrite becomes frozen in and can be re-heated subsequently (off-line) to 950° C. in order to form austenite particles in the grain interior.

The cast strip is thus cooled down during high-intensity cooling in accordance with the invention to an intermediate temperature of 900-1000° C., so that the temperature is rapidly reached in a direct way.

Rapid cooling to the intermediate temperature in accordance with the invention and holding at the maintenance temperature between the casting process and the coiling process have a positive effect, also with such methods, on the insensitivity of the cold strip obtained to ridging and orange peel formation, wherein hot-rolling between the casting process and the coiling process is omitted. In the case of such cast strip, which is produced, for example via a twin roll casting machine, the cast strip however with respect to the homogeneity of its micro-structure formation and its characteristic distribution is hot-rolled between the strip casting process and the coiling process in at least one pass. The density and precipitation rate of austenite particles are increased by the hot deformation, since structural defects are introduced into the micro-structure therewith. The exploitation of this mechanism for grain refinement depends upon optimum selection of the deformation conditions and the cooling and heating rates. In principle, rapid heating or cooling can be achieved on thin strip. Therefore, a strip produced by casting in the way according to the invention is particularly suitable for such thermo-mechanical treatment.

If hot-rolling is carried out with the method according to the invention, preferably a strip is cast to a thickness of 1-5 mm, particularly 2-3 mm, and then the cast strip is hot-rolled in-line with a reduction per pass of 5-60%, particularly 10-40%. The method according to the invention in this case enables temperature control of the strip to be selected with respect to hot-rolling carried out if necessary, so that temperature conditions, which are matched to the ductile behavior of the steel processed in each case or the desired characteristic combination of the strip obtained, can prevail during hot-rolling.

Including hot-rolling, therefore the following variants of the method according to the invention result:

Variant 1:

casting the cast strip;

hot-rolling the strip at a hot-rolling temperature of not less than 1180° C., typically

1180-1270° C. (initial cooling of the cast strip takes place in the course of hot-rolling);

cooling immediately following hot-rolling to a temperature of 900-1000° C. at a cooling rate of at least 150° C. per second, the intermediate and maintenance temperatures being equal to this temperature;

holding the strip at the relevant temperature of between 900-1000° C., particularly 950° C.±20° C. for at least 10 seconds.

As regards this first variant of the method according to the invention the cooling according to the invention and holding of the strip are carried out after hot-rolling. At the same time in this case cooling should begin as soon as possible after hot-rolling, in practice therefore within less than three, particularly within one second, after leaving the final hot-rolling stand. In this way the casting heat of the cast strip can be conducted (e.g., directly conducted) into the hot-rolling stage, so that not only high hot-rolling temperatures are possible, but also the energy consumption, necessary for controlling the temperature of the strip, is reduced to a minimum.

Variant 2:

casting the cast strip;

cooling to an intermediate temperature of 900-1000° C. at a cooling rate of at least 150° C. per second;

hot-rolling the strip at the intermediate temperature;

holding the strip at a maintenance temperature, which also lies between 900 and 1000° C., and particularly is substantially equal to the intermediate temperature or amounts to 950° C.±20° C., for at least 10 seconds.

With this variant of the invention cooling to the intermediate temperature is carried out before hot-rolling and holding at the maintenance temperature after the cast strip has been hot-rolled. By hot-rolling in the temperature range of 900-1000° C., additional dislocations, which serve as nucleus points for the austenite formation while being held subsequently at the maintenance temperature, are produced in the micro-structure of the hot-rolled strip.

Variant 3:

casting the cast strip;

cooling to an intermediate temperature of 900-1000° C. at a cooling rate of at least 150° C. per second;

holding the strip at a maintenance temperature, which also lies between 900-1000° C., and particularly is substantially equal to the intermediate temperature or amounts to 950° C.±20° C., for at least 10 seconds;

hot-rolling the strip at the intermediate temperature.

In accordance with this third variant cooling to the intermediate temperature and holding at the maintenance temperature are carried out before the cast strip is hot-rolled. Hot-rolling the micro-structure with the high density, produced by holding at the maintenance temperature carried out beforehand, of the austenite grains in the ferritic matrix leads to high dislocation density which on subsequent recrystallization results in a fine-grained structure. Such recrystallization can be due to suitable recrystallization annealing treatment, as it is performed as standard in the production of cold-rolled strip of the type under discussion.

Variant 4:

casting the cast strip;

cooling at a cooling rate of at least 150° C. per second to an intermediate temperature below 900° C., particularly in the range of 800° C.;

hot-rolling the strip at the intermediate temperature;

rapid heating of the strip to a maintenance temperature of 950° C.±50° C., particularly 950° C.±20° C.;

holding the strip at the maintenance temperature.

Hot-rolling in the range of temperatures less than 900° C., particularly in the range around 800° C., here takes place in the pure ferrite phase with a yield stress, which is lower in relation to rolling in the mixed phase. Thus, a higher deformation degree can be achieved with reduced energy consumption and less roll wear, if this is necessary and desirable.

High-intensity cooling in accordance with the invention in temperature ranges below 900° C. offers the possibility of rolling the cast strip at temperatures substantially below 800° C. or carrying out further thermal treatment at temperatures of less than 500° C., particularly less than 400° C.

The method according to the invention in the variants described above can be implemented particularly economically on such strip casting machines, wherein casting, hot-rolling carried out if necessary and coiling as well as the steps according to the invention carried out between the casting process and the coiling process of cooling to the intermediate temperature and holding at the maintenance temperature are performed in a sequence of steps continuously one after the other.

The effects exploited by the invention however also offer the possibility of carrying out the individual work steps of the method according to the invention discontinuously. This can prove advantageous for example if corresponding plant technology is available or logistical reasons favor execution of the work steps at different times. This results in the following fifth variant of the invention.

Variant 5:

casting the cast strip;

cooling at a rate of at least 150° C. per second to an intermediate temperature below 900° C., particularly below 800° C.;

cooling to less than 500° C., particularly to ambient temperature;

rapid heating to a hot-rolling end temperature;

hot-rolling of the strip at the hot-rolling temperature;

rapid heating of the strip to a maintenance temperature of 950° C.;

holding the strip at the maintenance temperature.

According to this fifth variant of the invention provision is made for the cast strip in the course of cooling in accordance with the invention to be cooled to an intermediate temperature of less than 900° C., particularly less than 800° C., this cooling possibly going as low as ambient temperature. Subsequently the cast strip is then re-heated to the maintenance temperature. Subsequently in this context means that further work steps, such as for example hot-rolling at a specific temperature, storage, dividing into plates etc., may be carried out between the cooling and the holding steps.

Furthermore, it is possible following casting to cool the cast strip to ambient temperature and, at a subsequent point in time, heat it to the optimum temperature for hot-rolling and then bring it to the maintenance temperature and hold it there for the required period.

If when the method according to the invention is carried out, intermediate temperatures in the range of 800-900° C. are reached, when re-heating to the maintenance temperature, this temperature range should be passed through rapidly due to the reasons already mentioned. Therefore, one advantageous embodiment of the invention provides for re-heating to the maintenance temperature, starting from the respective intermediate temperature, to take place in 1-5 seconds, particularly in 2-3 seconds.

If the intermediate temperatures reached in the course of cooling lie substantially below 800° C., especially in the range of the ambient temperature or slightly above this, the strip must be re-heated for hot-rolling sufficiently rapidly due to the reasons already mentioned. Therefore, the invention provides, starting from the low intermediate temperature, for the strip to be re-heated within 200 seconds, particularly within 100 seconds, to the specific hot-rolling temperature, which will typically be 700-800° C. If heating to 800° C. is too slow, unwanted carbides may precipitate. These lead to a premature decrease in supersaturation and thus to a substantially reduced density of austenite particles with the result that the grain refinement aimed at by the invention is not achieved.

The invention therefore provides a method, which offers the possibility, while avoiding expensive production steps, of making a product that is competitive regarding both its characteristics and its price. The special advantage of the method according to the invention lies in the fact that cold strip characterized by a homogeneous appearance and a scale-free surface can be produced. The latter is achieved by the fact that any scale adhering to the cast strip is already removed as much as possible during the high-intensity cooling according to the invention, so that at worst minimum surface damage due to scale still present on the strip is caused during hot-rolling carried out if necessary.

With the method according to the invention therefore expensive descaling leading to interruption of the continuous production sequence, is dispensed with. Also, expense, which is still required with the prior art for the necessary batch type annealing of the cold-rolled material, can be saved by the method according to the invention. The possibility, during the course of the method according to the invention, of cooling to temperatures below 800° C. further permits, for example, the cast or hot-rolled strip to be trimmed at temperatures, e.g. 600° C., wherein flatness problems can be generally avoided. The method according to the invention permits the hot deformation degree to be increased in the strip casting line, due to the generally freely variable temperature control, via for example a second roll stand or a smaller working roll diameter, as a result of which cold strip produced according to the invention can possess better deep/stretch-drawing capacity compared to such cold strip which has been made via the usual production route. The rapid temperature changes necessary for implementing the method according to the invention can be realized in this case by using the strip-casting technology, since the minimum thickness of the cast strip allows sufficiently rapid temperature changes over the entire cross section of the strip. Exemplary embodiments:

In each case a ferritic steel, which contained (in wt. %) 0.043% C, 0.25% Si, 0.36% Mn, 0.021% P, 0.002% S, 16.23% Cr, 0.49% Ni, was the basis of the exemplary embodiments described below, carried out to prove the effectiveness of the invention (Trials I-Iv).

In a casting rolling plant of conventional design a cast strip was produced in each case from a suitable molten steel, the cast strip hot-rolled to form a hot strip and the hot strip coiled. The strip-casting plant used for this comprised a twin roll casting machine, a hot-rolling stand arranged in the conveyance direction of the cast strip following the casting machine in-line and a coiling device arranged in the conveyance direction behind the hot-rolling stand. Depending upon the particular trial procedure, high-intensity water cooling equipment, inductively-operating ovens and electric temperature maintenance furnaces were also used.

After the thermal treatment according to the invention the hot strip obtained had a fine-grained structure, in which contrary to micro-structures conventionally described as “fine” a plurality of particles (martensite, residual austenite, carbide) are found in a relatively large ferritic grain (=matrix). Accordingly the micro-structure is altogether more fine, in itself however also more inhomogeneous than conventionally fine-grained micro-structures. Characteristic of the micro-structure of the strip produced according to the invention is therefore the high number of particles per grain.

Each hot strip produced according to the invention in trials I-IV using the casting rolling plant was subsequently processed in a conventional way including batch type annealing, pickling, cold-rolling without intermediate annealing, bright annealing and skin-pass rolling.

Test specimens were produced from the cold strip obtained in such a way with a total deformation degree of 70%. None of the test specimens showed orange peel or ridging.

Trial I:

In a first variant of the method according to the invention the thickness of the cast strip was 3 mm. After the cast strip leaving the casting gap of the twin roll casting machine had reached a strip temperature of 1180° C., high-intensity water cooling took place. The cast strip was cooled within 2 seconds to an intermediate temperature of 950° C.

The cast strip cooled in such a manner was then held without interruption in a continuous production sequence at a maintenance temperature, which in this case was equal to the intermediate temperature, in an inductive heating oven for a period of 10 seconds.

The cast strip, thermally-treated in this way according to the invention, was then hot-rolled to a strip thickness of 2.5 mm.

The strip cooled down on the run-out roller table following the hot-rolling stand to a coiling temperature of approximately 550° C., before it reached the coiling device, where it was coiled to a coil.

The hot strip obtained in such a way had a columnar grain structure (approx. 100 μm wide and 500 μm long) with an equiaxed strip central area (grain size 150 μm). The grain boundaries were occupied by a thin seam of martensite and carbides. Recrystallized areas with a grain size of 20 μm were found in the grain interior. Also, finely distributed isolated particles, which consisted of carbides, martensite and residual austenite, were present in the micro-structure. The particle density was typically 15-25 particles per grain.

Trial II:

In a second trial a 2.8 mm thick strip was produced from the molten steel with the alloy indicated above. The cast strip was held in an inductive heating oven at a temperature of 1200° C. and then hot-rolled at this temperature to a strip thickness of 2.1 mm.

Immediately after hot-rolling, high-intensity water cooling took place. In this case the strip travelling at approx. 1 meter per second was cooled within one second to an intermediate temperature of 950° C. The strip then reached a run-out roller table, whose first section associated with the hot-rolling stand, was equipped with a hood over a length of 15 meters, which guaranteed that the strip was held in this first section at a substantially constant temperature for 15 seconds. Subsequently, the strip still on the run-out roller table cooled down to a coiling temperature of approximately 500° C. at which it was coiled to a coil.

The micro-structure of the hot strip obtained in the second trial had the same columnar grain structure (approx. 100 μm wide and 500 μm long) with an equiaxed strip central area (grain size 150 μm) as the micro-structure of the hot strip obtained in the first trial. Also, in this case the grain boundaries showed a thin seam occupied by martensite and carbides. Likewise, recrystallized areas with an average grain size of 20 μm were found in the grain interior. Also, finely distributed isolated particles, which also as with the strip obtained in the first trial consisted of carbides, martensite and residual austenite, were present in the micro-structure. The particle density was typically 20-30 particles per grain.

Trial III:

In a third trial, a 3 mm thick strip was cast. After the cast strip had reached a temperature of 1180° C., high-intensity water cooling began, the strip being cooled down within 3 seconds to an intermediate temperature of 780° C. The cast strip cooled in this way was then kept hot in an inductive heating oven, heated to a hot-rolling temperature of 800° C. and then hot-rolled at this hot-rolling temperature to a strip thickness of 2.5 mm. The strip then cooled down on the run-out roller table to a coiling temperature of approx. 550° C. and coiled at this temperature.

Specimen plates were divided at ambient temperature from the strip obtained in such a way. These were then heated inductively to 800° C. and then to 950° C. within a period of 15 seconds. The period for heating between 800° C. and 950° C. lasted 2 seconds.

The strip was thereupon held for 20 seconds at a maintenance temperature of 950° C. by means of a maintenance furnace. Subsequently, air cooling took place.

The micro-structure of such thermally treated hot strip specimen plates likewise showed a columnar grain structure (approx. 100 μm wide and 500 μm long) with an equiaxed strip central area (grain size 150 μm). At the grain boundaries also here a thin seam was occupied by martensite and carbides. Again recrystallized areas with a grain size of 20 μm were found in the grain interior and finely distributed isolated particles, which consisted of carbides, martensite and residual austenite, were present in the micro-structure. The particle density here was typically 40-60 particles per grain.

Trial IV:

Similarly to trial III a 3 mm thick strip was produced, which on reaching a strip temperature of 1180° C. was cooled at high-intensity until an intermediate temperature of 780° C. was reached. Deviating from trial III, however, not only the maintenance temperature holding step but also the hot-rolling step was carried out off-line.

For this purpose, plates were divided from the cast strip after it had cooled down to ambient temperature and these plates were inductively heated from ambient temperature within 30 seconds to a hot-rolling temperature of 800° C., wherein they were hot-rolled to a strip thickness of 2.4 mm. After repeated cooling of the hot-rolled plates they were re-heated within 3 seconds to a maintenance temperature of 950° C.

The re-heated strip was held for 20 seconds at the maintenance temperature by means of a maintenance furnace. Subsequently, the strip was air cooled.

Also, in this exemplary embodiment the micro-structure of the hot-rolled plates showed a columnar grain structure (approx. 100 μm wide and 500 μm long) with an equiaxed strip central area (grain size 150 μm) after being held at the maintenance temperature, wherein the grain boundaries also here had a thin seam occupied by martensite and carbides and recrystallized areas with a grain size of 20 μm were found in the grain interior. Similar to other trials, finely distributed isolated particles, consisting of carbides, martensite and residual austenite, were also present in the micro-structure. The particle density was typically 40-60 particles per grain. 

1. Method of making a cold strip with a ferritic structure comprising, casting molten steel which forms a ferritic structure on cooling into a cast strip, coiling the cast strip cold-rolling the cast strip to form the cold strip, cooling the cast strip between casting and coiling from a starting temperature not lower than 1180° C., at a cooling rate of at least 150° C. per second, to a maximum intermediate temperature of 1000° C., and holding the cast strip for at least 10 seconds at a maintenance temperature of between 900 and 1000° C.
 2. Method according to claim 1, wherein the starting temperature of the cooling ranges from 1180 to 1270° C.
 3. Method according to claim 1, wherein the molten steel is composed as follows (in wt. %): Cr: 10.5-18%, C: ≦0.08%, Si: ≦1%, Mn: ≦1.5% Ni: ≦1.00% P: ≦0.04% S: ≦0.015% Remainder iron and unavoidable impurities.
 4. Method according to claim 1, wherein the cooling rate is 150-250° C. per second.
 5. Method according to claim 1, wherein the intermediate temperature is 900-1000° C.
 6. Method according to claim 5, wherein the intermediate temperature is equal to the maintenance temperature.
 7. Method according to claim 1, further comprising hot rolling the cast strip.
 8. Method according to claim 7, wherein the cooling the cast strip and holding the cast strip are carried out after hot-rolling.
 9. Method according to claim 8, wherein cooling of the strip starts within less than three seconds, after hot-rolling.
 10. Method according to claim 7, wherein cooling the cast strip to the intermediate temperature and holding the cast strip at the maintenance temperature are carried out before hot-rolling the cast strip.
 11. Method according to claim 7, wherein cooling to the intermediate temperature is carried out before hot-rolling and holding at the maintenance temperature is carried out after hot-rolling the cast strip.
 12. Method according to claim 1, wherein the cast strip on cooling reaches an intermediate temperature of less than 900° C., and is subsequently re-heated to the maintenance temperature.
 13. Method according to claim 7, wherein hot-rolling is carried out at the intermediate temperature of less than 800° C.
 14. Method according to claim 12, wherein re-heating takes place within 1-5 seconds.
 15. Method according to claim 12, wherein casting, coiling, cooling carried out between the casting process and the coiling process to the intermediate temperature and holding at the maintenance temperature are performed in a discontinuous production sequence.
 16. Method according to claim 15, wherein on cooling, the strip reaches a temperature of less than 500° C.
 17. Method according to claim 16, wherein following casting, the strip is cooled down to ambient temperature.
 18. Method according to claim 16, wherein the strip is heated for hot-rolling within 200 seconds, to a hot-rolling temperature.
 19. Method according to claim 12, wherein the casting, coiling, cooling carried out between the casting process and the coiling process to the intermediate temperature and holding at the maintenance temperature are performed in a continuous production sequence.
 20. Method according to claim 1, further comprising casting the strip with a twin roll casting machine.
 21. Method according to claim 1, wherein the maximum thickness of the cast strip is 6 mm.
 22. Method according to claim 1 further comprising hot-rolling the cast strip in-line. 